I started with Google play but quickly switched to amazon. Then the problem was you needed to have internet access at all time to read the books. It clashed with me taking the underground rail a lot since there is no mobile internet there. Don't know if Google fixed that since then or not. I know I will stay with amazon...
I have Kindle,Google Play and Nook. Google Play the reader is not as friendly as the Kindle. The Nook reader I use only when I find a good deal on it, which is not often. My 7 and 10 year old children even use the Kindle. They adjust their own settings and made their own book collection on it.